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Reactions of Alkenes: Addition Reactions Hydrogenation of Alkenes – addition of H-H (H2) must be catalyzed by metals such as Pd, Pt, Rh, and Ni. H H H C + C Pd/C H H EtOH H H C-C π-bond = 243 KJ/mol H-H = 435 KJ/mol H C C H H H H°hydrogenation = -136 KJ/mol H C-H = 2 x -410 KJ/mol = -142 KJ/mol • The catalysts is not soluble in the reaction media, thus this process is referred to as a heterogenous catalysis. • The reaction takes places on the surface of the catalyst. Thus, the rate of the reaction is proportional to the surface area of the catalyst. Carbon-carbon -bond of alkenes and alkynes can be reduced to the corresponding saturated C-C bond. Other -bond bond such as C=O (carbonyl) and CN are not easily reduced by catalytic hydrogenation. The C=C bonds of aryl rings are not easily reduced. O O H2, PtO2 ethanol O C5H11 OH H2, Pd/C CH3(CH2)16CO2H Linoleic Acid (unsaturated fatty acid) Steric Acid (saturated fatty acid) O O OCH3 H2, Pd/C OCH3 ethanol C H2, Pd/C N ethanol C N Table 6.1 (pg 228): Heats of Hydrogenation of Some Alkenes Alkene H2C=CH2 H H H3C H monosubstituted H H° (KJ/mol) 136 125 - 126 H 117 - 119 H3C CH3 H CH3 disubstituted H3C H3C H 114 - 115 H 116 - 117 H3C H H3C H H3C CH3 H3C CH3 H3C CH3 trisubstituted tetrasubstituted 112 110 Stereochemistry of Alkene Hydrogenation Mechanism: H H H2C CH2 H H H2C CH2 H2C CH2 H2 H H H H C C H H H H H C H H C H The addition of H2 across the -bond is syn, i.e., from the same face of the double bond CH3 CH3 H H2, Pd/C EtOH H CH3 CH3 H CH3 H syn addition of H2 CH3 Not observed Electrophilic Addition of Hydrogen Halides to Alkenes C-C -bond: H°= 368 KJ/mol C-C -bond: H°= 243 KJ/mol -bond of an alkene can act as a nucleophile!! Electrophilic addition reaction H H Br C C H + H-Br H nucleophile H H C C H H H electrophile Bonds broken C=C -bond 243 KJ/mol H–Br 366 KJ/mol Bonds formed H3C-H2C–H -410 KJ/mol H3C-H2C–Br -283 KJ/mol calc. H° = -84 KJ/mol expt. H°= -84 KJ/mol Reactivity of HX correlates with acidity: slowest HF << HCl < HBr < HI fastest Regioselectivity of Hydrogen Halide Addition: Markovnikov's Rule R R R H C R C R C C H C H C R H H-Br Br H R C C H H H H H-Br Br H R C C H R H H H-Br Br H R C C R R H H Br + R C C H H H none of this + + H Br R C C H R H none of this H Br R C C R R H none of this H R C C R' H H-Br Br H R C C R H H + H Br R C C R' H H Both products observed Mechanism of electrophilic addition of HX to alkenes Mechanistic Basis for Markovnikov's Rule: Markovnikov’s rule can be explained by comparing the stability of the intermediate carbocations For the electrophilic addition of HX to an unsymmetrically substituted alkene: • The more highly substituted carbocation intermediate is favored (hyperconjugation). • The more highly substituted carbocation is formed faster than the less substituted carbocation and also thermodynamically more stable. Carbocation Rearrangements in Hydrogen Halide Addition to Alkenes - In reactions involving carbocation intermediates, the carbocation may sometimes rearrange if a more stable carbocation can be formed by the rearrangement. These involve hydride and methyl shifts. H C H3C C H3C Cl H H-Cl C H C H3C H3C H H H H C C H H + H ~ 50% expected product H C H3C C H3C Cl CH3 H C H H-Cl H3C C C Cl H H3C H C C CH3 H H H H ~ 50% H C H3C C H3C H3C H + C H3C H3C H H C C Cl H H Note that the shifting atom or group moves with its electron pair. A MORE STABLE CARBOCATION IS FAVORED. Free-radical Addition of HBr to Alkenes H3CH2C H3CH2C R R R H H C H C H C R C R C R C C H C H H H-Br Br H H3CH2C C C H H H H H-Br Br H H3CH2C C C H H H peroxides (RO-OR) H-Br C H C H C R C R' H ROOR (peroxides) H H-Br ROOR H H-Br ROOR H H-Br ROOR + + H Br H3CH2C C C H H H none of this H Br H3CH2C C C H H H Polar mechanism (Markovnikov addition) Radical mechanism (Anti-Markovnikov addition) none of this Br H R C C H H H none of this Br H R C C H R H none of this Br H R C C R R H none of this Br H R C C R H H + + H Br R C C H H H H Br R C C H R H + H Br R C C R R H + H Br R C C R' H H Both products observed The regiochemistry of HBr addition is reversed in the presence of peroxides. Peroxides are radical initiators - change in mechanism 136 The regiochemistry of free radical addition of H-Br to alkenes reflects the stability of the radical intermediate. H H R C• R C• H Primary (1°) R R C• R < Secondary (2°) R < Tertiary (3°) 137 Acid-Catalyzed Hydration of Alkenes - addition of water (H-OH) across the -bond of an alkene to give an alcohol; opposite of dehydration H3C C H3C CH2 H2SO4, H2O H3C H3C H3C C OH This addition reaction follows Markovnikov’s rule The more highly substituted alcohol is the product and is derived from The most stable carbocation intermediate. Reactions works best for the preparation of 3° alcohols 138 Mechanism is the reverse of the acid-catalyzed dehydration of alcohols: Principle of Microscopic Reversibility 139 Thermodynamics of Addition-Elimination Equlibria H3C H2SO4 C CH2 + H2O H3C Bonds broken C=C -bond 243 KJ/mol H–OH 497 KJ/mol H3C C H3C H3C OH Bonds formed H3C-H2C–H -410 KJ/mol (H3C)3C–OH -380 KJ/mol calc. H° = -50 KJ/mol G° = -5.4 KJ/mol H° = -52.7 KJ/mol S° = -0.16 KJ/mo How is the position of the equilibrium controlled? Le Chatelier’s Principle - an equilibrium will adjusts to any stress The hydration-dehydration equilibria is pushed toward hydration (alcohol) by adding water and toward alkene (dehydration) by 140 removing water The acid catalyzed hydration is not a good or general method for the hydration of an alkene. Oxymercuration: a general (2-step) method for the Markovnokov hydration of alkenes H H C C4H9 H 1) Hg(OAc)2, H2O C H Hg(OAc) C H H O C H3C C C4H9 H Ac= acetate = OH O 2) NaBH4 OH C C4H9 H C H H NaBH4 reduces the C-Hg bond to a C-H bond 141 Hydroboration-Oxidation of Alkenes - Anti-Markovnikov addition of H-OH; syn addition of H-OH CH3 1) B2H6, THF 2) H2O2, NaOH, H2O H HO CH3 H Stereochemistry of Hydroboration-Oxidation Mechanism of Hydroboration-Oxidation Step 1: syn addition of the H2B–H bond to the same face of the -bond in an anti-Markovnikov sense; step 2: oxidation of the B–C bond by basic H2O2 to a C–OH bond, with retention of stereochemistry 142 Addition of Halogens to Alkenes X2 = Cl2 and Br2 X2 X X (vicinal dihalide) C C C C alkene 1,2-dihalide Stereochemistry of Halogen Addition - 1,2-dibromide has the anti stereochemistry Br Br + + Br2 Br Br not observed CH3 Br Br2 H CH3 Br 143 Mechanism of Halogen Addition to Alkenes: Halonium Ions - Bromonium ion intermediate explains the stereochemistry of Br2 addition 144 Conversion of Alkenes to Vicinal Halohydrins "X-OH" X OH C C C C alkene halohydrin X2, H2O X + HX OH anti stereochemistry Mechanism involves a halonium ion intermediate 145 For unsymmterical alkenes, halohydrin formation is Markovnikov-like in that the orientation of the addition of X-OH can be predicted by considering carbocation stability CH3 Br + more + charge on the more substituted carbon H2O adds in the second step and adds to the carbon that has the most + charge and ends up on the more substituted end of the double bond CH3 HO Br2, H2O CH3 + HBr H Br Br adds to the double bond first (formation of bromonium ion) and is on the least substituted end of the double bond 146 Organic molecules are sparingly soluble in water as solvent. The reaction is often done in a mix of organic solvent and water using N-bromosuccinimide (NBS) as he electrophilic bromine source. O + N Br O O OH DMSO, H2O Br N H + O Note that the aryl ring does not react!!! Epoxidation of Alkenes - Epoxide (oxirane): threemembered ring, cyclic ethers. Reaction of an alkene with a peroxyacid: peroxyacetic acid O H3C O O peroxyacetic acid O H3C O H H3C OH acetic acid HO OH peroxide H O OH O H3C O + O Stereochemistry of the epoxidation of alkenes: syn addition of oxygen. The geometry of the alkene is preserved in the product Groups that are trans on the alkene will end up trans on the epoxide product. Groups that are cis on the alkene will end up cis on the epoxide product. H H R R H3CCO3H R R H trans-alkene O H R R cis-epoxide cis-alkene H H H3CCO3H H O R H R trans-epoxide Ozonolysis of Alkenes - oxidative cleavage of an alkene to carbonyl compounds (aldehydes and ketones). The and -bonds of the alkene are broken and replaced with C=O double bonds. C=C of aryl rings, CN and C=O do not react with ozone, CC react very slowly with ozone Ozone (O3): 3 O2 electrical discharge + O 2 O3 1) O3 2) Zn O O + 1) O3 2) Zn _ O H H + O C H O 1) O3 2) Zn O O H O Introduction to Organic Chemical Synthesis Synthesis: making larger, more complex molecules out of less complex ones using known and reliable reactions. devise a synthetic plan by working the problem backward from the target molecule OH ?? H2SO4 H2, Pd/C OH ?? 152 CH3 CH3 Br ?? H Br